The goal of this project is to identify and examine longitudinally, with positron emission tomography, the earliest alterations in cerebral physiology associated with the onsets of Alzheimer's disease and of parkinsonian dementia. PET determined measures of cerebral glucose metabolism, blood flow, muscarinic cholinergic receptor densities, and presynaptic cholinergic markers will be determined in cross-sectional and longitudinal studies of controls and patients with dementing diseases.